Mixing of immiscible liquids and/or mixing a particulate solid, herein referred to generically as a “powder,” with a liquid or with another powder are important requirements in many applications and industries. Examples of mixing two immiscible liquids are found throughout the chemical, petroleum, mining, and pharmaceutical industries. These include dispersing and emulsifying food components when preparing mayonnaise, or mixing latex with water to make water based paints.
Powders are mixed with liquids during the manufacture of paints, inks, fillers, caulks, composite plastics, toothpastes, greases modified with metal powder, concrete, and some foodstuffs such as when dry ingredients are mixed with batter.
Examples of manufacturing processes which require mixing of two or more dry powders include mixing dry pigment blends, mixing sand with cement before adding water to make concrete, mixing granulated sugar with flour or with powdered sugar or cocoa powder in the manufacture of food products, and mixing of pharmaceuticals.
Mixing of liquids with liquids, liquids with powders, and powders with powders is especially critical when the volume of one of the liquids or powders is very small relative to the volume(s) of the other or others, for example when adding a catalyst or a small percentage additive to a resin system.
Many different types of mixing apparatus are commonly used for mixing immiscible liquids, mixing powders with liquids, and/or mixing powders with other powders. Most mixers fall into one of two basic categories: a “continuous” mixer or a “batch” mixer. In a continuous mixer, the components are continuously added in appropriate ratios to an “input” of the mixer. The components are mixed as they flow through the mixer, and then are dispensed from an “output” of the mixer. This process is continued until the desired quantities of components have been mixed.
In a batch mixer, the full quantities of all of the components to be mixed are placed into the interior of a container at the beginning of the mixing process, and the components remain in the container until the mixing is complete, after which the entire contents of the container are removed. FIG. 1A illustrates a batch mixer which includes a large capacity mixing container 100 typically capable of mixing a load weighing between 100 kg and 10,000 kg. The batch mixer of FIG. 1A is an example of a “vertical shaft” mixer which blends the contents of the mixing container 100 by using a rotating “high speed disperser blade” or “impeller” (106 in FIG. 1B) suspended on a vertical shaft 102 within the interior of the mixing container 100 and driven by a motor 104. FIG. 1B illustrates the motor, shaft, and disperser blade of a similar model of vertical shaft batch mixer.
FIG. 1C illustrates a typical flow pattern for the contents of a vertical shaft batch mixer of the type illustrated in FIG. 1A. The impeller 106 is typically designed to propel the contents of the container 100 in both horizontal and vertical directions, thereby creating convection flow within the container which tends to mix all portions of the contents together. In this example, the impeller 106 is a “high sheer” blade, which is designed to create high sheer stresses within a viscous mixture, thereby helping to break up large droplets and/or any clumps of powder granules.
FIG. 2 illustrates a typical medium speed vertical shaft batch mixer used, for example, to mix mortar or paste. This design includes two impellers 106 driven by two parallel shafts 102. Mixer sizes for this general style can range from small laboratory models to large production models, with container capacities between about 2 kg and 10,000 kg.
FIG. 3 illustrates another style of vertical shaft batch mixer, which operates at a low or medium speed, and is typically used for mixing viscous materials such as mortar, bread or pastry dough, or paste. Other examples of such viscous materials include mixing carbon black and/or other powders into latex or synthetic rubber bases to create useful “rubber” materials for tires, hoses, plastics, etc. Since these “liquids” are often too viscous to create convection flow, this style of mixer, referred to herein as a “planetary” mixer, includes a complex “planetary” impeller 106 having a plurality of distributed mixing elements which sweep through most of the container volume as the vertical shaft 102 rotates. Sizes for this type of mixer can typically range from small kitchen models up to industrial models with a 10,000 kg or 15,000 kg capacity. FIG. 4 illustrates the design of a typical planetary impeller 106 which might be used in a mixer such as FIG. 3.
The vertical shaft batch mixers illustrated in FIGS. 1A through 4 are typically used for mixing immiscible liquids and/or mixing a powder with a liquid (note that herein the term “liquid” is used to refer to both low viscosity liquids such as water and high viscosity liquids such as dough, certain resins, and polymer bases). Such mixing can include multi-step mixing, for example using a Hockmeyer or Kohler type, high RPM, high sheer dispersing mixer having a serrated blade disperser for first dissolving liquid additives into a base liquid, and then dispersing dry solid materials (e.g. pigment, fillers, or reactants) into the liquid.
In general, vertical shaft batch mixers are not satisfactory for mixing two dry powders together, since dry powders lack the fluid viscosity necessary for establishing the convective flow illustrated in FIG. 1C, or even the local flow required for a vertical shaft planetary mixer. Hence, when it is necessary to mix two powders in a batch mixer, a “horizontal” batch mixer is frequently used. In a horizontal batch mixer, either an agitator or the container itself is tumbled or rotated about a horizontal axis, so as to lift the contained powders or other contents and cause them to be impelled by gravity to mix with each other.
An example of a horizontal shaft batch mixer is illustrated in FIG. 5, wherein the mixing container 100 is shaped roughly as a “V,” and is rotated about a horizontal axis. In some models of horizontal batch mixer, the interior walls of the container 100 include baffles or fins which further help to lift and mix the contents as the container is rotated. Sizes for this type of mixer can range typically from small laboratory models up to industrial models having a container capacity of about 20,000 kg or more.
Another type of “closed vessel” horizontal mixer is illustrated in FIG. 6, and an “open-vessel” horizontal mixer is illustrated in FIG. 7. For these two mixer designs, the container is stationary, and the contents are mixed by a horizontally rotating agitator 106. Due to the horizontal geometry, the agitator 106 is able to vertically lift portions of the container contents as the agitator rotates, after which the lifted contents mix as they fall back toward the bottom of the container 100.
Many styles of horizontal batch mixer are able to mix powders with almost any other material, including a second powder, a viscous “liquid” such as bread dough, or a non-viscous liquid such as water. Horizontal mixers can be useful for multi-step mixing processes such as mixing concrete, where first two powders (cement and gravel) must be mixed, and then the combined powders must be mixed with water.
The primary difficulty which must be overcome by a mixer in mixing immiscible liquids is to minimize the sizes of the droplets within the resulting emulsion. Initially, the mixer will tend to separate the two liquids into interspersed regions, and larger regions will continue to be separated into smaller regions until the mixture becomes an emulsion of suspended liquid droplets. However, depending on properties of the liquids such as their viscosities and surface tensions, once the droplets have been reduced to a certain size, further droplet size reduction becomes difficult as droplets of each liquid collide and coalesce with each other into larger droplets as they move through the mixture.
In general, for a mixer to mix a powder with a liquid or with another powder, the mixer must overcome at least three difficulties. First, the granules of a powder do not naturally flow in the manner of a liquid. Second, in a manner which resembles the droplets formed by immiscible liquids, the granules of a powder tend to aggregate together and form “clumps,” such that the clumps may tend to remain intact even when they are dispersed throughout the liquid or second powder in the proper weight percentage. Third, the granules of a powder can tend to adhere to the walls of a container and to the surfaces of an agitator, so that some fraction of the powder remains unmixed.
The failure of powders to flow like liquids generally excludes the use of vertical shaft batch mixers when mixing two dry powders together, as discussed above. Instead, other mixer styles such as horizontal batch mixers are typically used. When mixing a powder with a liquid in a continuous mixer, it is sometimes necessary to add more of the liquid phase than would be desirable, simply to reduce the viscosity and allow the mixture to flow through the mixing tube.
When immiscible liquid droplets and/or particle clumping are a concern, a batch mixer is generally used, since it is difficult for a continuous mixer to address the problem of droplet size and particle clumping. When mixing immiscible liquids or mixing a powder into a viscous liquid, the problems of droplets and/or powder clumping are sometimes addressed in a vertical shaft batch mixer by using a “high sheer” impeller (see FIG. 1C), which is designed to generate high sheer forces within the mixture in an attempt to break up droplets and/or powder clumps. However, this can lead to heating of the mixture, with a consequent loss of viscosity and/or damage to the mixture, so that active cooling of the mixing container 100 is sometimes needed to prevent damage to the mixture and to maintain sufficient viscosity for the sheer forces to be effective in breaking up the droplets and/or clumps. An example where loss of viscosity is of concern is in resinous liquids, which can experience rapid declines in viscosity as their temperature increases.
Generally speaking, for each application and each industry in which mixing of immiscible liquids or mixing of a powder with a liquid or with another powder is required, an appropriate style of mixer and a time and energy requirement for proper mixing are known. For many of these industries, the energy consumed and the mixing time required are important contributors to the total cost of a production process. Quality and degree of mixing are also highly important. Apparatus and methods for reducing the required time and energy while improving the quality and degree of mixing would therefore be highly desirable.
What is needed, therefore, is an apparatus and method for reducing the time and energy required for mixing immiscible liquids and/or mixing a powder with a liquid or with another powder.